Postfix PostgreSQL Howto

The Postfix pgsql map type allows you to hook up Postfix to a
PostgreSQL database. This implementation allows for multiple pgsql
databases: you can use one for a
virtual(5) table, one for an
access(5) table, and one for an
aliases(5) table if you want. You
can specify multiple servers for the same database, so that Postfix
can switch to a good database server if one goes bad.

Busy mail servers using pgsql maps will generate lots of
concurrent pgsql clients, so the pgsql server(s) should be run with
this fact in mind. You can reduce the number of concurrent pgsql
clients by using the Postfix
proxymap(8) service.